The microprocessor-based thermostats which have become very prevalent provide substantial advantages of safe and accurate control of furnaces and air conditioning. In addition, the use of microprocessors allows easy addition of various features not otherwise easy to provide and at the same time improves the ease with which the owner can select the thermostat actions and parameters desired.
In these types of thermostats there are two main systems for providing power to the thermostat electronics. In one type, power is "stolen" when the relay or thyristor which controls furnace operation is open, from the 24 VAC power transformer which operates the furnace components. This power drives a DC power supply which charges a battery. The battery then operates the thermostat components. A second design uses simple replaceable dry cells as the power source for the microprocessor. Because the market demands it, it is necessary to extract a full season's use out of one set of dry cells. Therefore, the microprocessor itself as well as any other circuitry powered by the replaceable dry cells must have very low power draw.
Because of this requirement for low power draw, it is necessary for the microprocessor to provide very low power output signals to switch the power to the HVAC control. However, the use of low power output signals makes them vulnerable to noise which may cause power to be briefly supplied to the HVAC control. At the very least, such noise pulses can provide short power spikes which actuate the internal HVAC control relays briefly, and in a noisy situation, can lead to shortened control life. Certain types of controls also have timed sequences which start upon first application of power, and in this case each noise pulse will cause at least the first part of the sequence to occur. This is obviously undesirable.
In these controls it is also desirable to use an inexpensive thyristor such as a triac as the switching element to control the power to the HVAC control rather than the relatively expensive latching relay which has been used in the past. The term "thyristor" is used hereafter to refer to any semiconductor device used for switching AC power. Power to operate the thyristor control circuitry is taken from across the thyristor. It is possible for this thyristor control circuitry to sometimes take a state which makes the thyristor conducting when power is first applied to them. It is important that failures in thermostat operation always lead to removing power from the HVAC control. This is because particularly in the operation of a furnace, continuous operation of the controlled device is much more dangerous than simply shutting it down. So in situations where a short power failure occurs, upon power being restored, the thyristor must not assume its conducting state.
A further safety-related problem involves complete or partial operating failure of the microprocessor. It is not easy to detect such a condition, but use of a watchdog-type of output port to produce the signal calling for conduction by the thyristor can substantially reduce the likelihood of microprocessor failure locking the thyristor in a conducting condition. Such a microprocessor port calls for conduction by the thyristor with a square wave output signal oscillating at some predetermined frequency generated by or during the execution of the programmed instructions. Any signal having a constant voltage or a frequency different from the predetermined value is used to request that the thyristor not conduct. An example of such a watchdog signal pattern in the burner control field is shown in U.S. Pat. No. 4,865,538. The theory is that it is unlikely that an improperly operating microprocessor will usually be incapable of producing the predetermined frequency as the output signal, and in fact this is a reasonable expectation.
U.S. Pat. No. 5,151,854 issued Sep. 29, 1992; having as joint applicant the applicant in this application, having the same assignee, and entitled Integrated Low Voltage Detect and Watchdog Circuit, discloses a circuit of which part is very similar to the detector circuit portion of the circuit to be described.